The present invention relates generally to exhaust sensors. More particularly, the present invention relates to a means for sealing a sensor in a mat supported exhaust sensor.
Exhaust sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, exhaust sensors have been used for many years in automotive vehicles to sense the presence of oxygen in exhaust gases, i.e., to sense when an exhaust gas content switches from rich to lean or lean to rich. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the exhaust sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and management of exhaust emissions.
A conventional stoichiometric exhaust sensor typically consists of an ionically conductive solid electrolyte material, a porous electrode on the sensor""s exterior exposed to the exhaust gases with a porous protective overcoat, and a porous electrode on the sensor""s interior surface exposed to a known oxygen partial pressure. Sensors typically used in automotive applications use a yttria stabilized zirconia-based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:   E  =            (              RT                  4          ⁢          F                    )        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )            
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
pO2ref=oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the exhaust gas
Due to the large difference in oxygen partial pressures between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric exhaust sensors indicate qualitatively whether the engine is operating fuel rich or fuel lean, without quantifying the actual air to fuel ratio of the exhaust mixture.
In conventional automotive exhaust applications, the body of an exhaust sensor utilizing talc for the dual purposes of sealing and sensor support has been prone to infiltration by exhaust fumes through undesirable internal air leaks in the exhaust sensor. Referring to FIG. 1, a prior art exhaust sensor has an upper ceramic disk 30 and a lower ceramic disk 32, between which is disposed a dual talc pack 34. The dual talc pack 34 supports the sensor element 6, and seals the exhaust sensor components from the exhaust gases. This configuration, however, has limited usefulness because the ceramic disks 32, 34 are generally about 5 millimeters thick or greater. This space requirement increases the overall required size of the exhaust gas sensor. Additionally, ceramic disks require the use of gaskets or other means for preventing stress points on the ceramic. Finally, since the dual talc pack 34 serves as the only means of supporting the sensor element 6, the talc seal is subject to breakdown due to mechanical disruption.
Sealing the internal components of an exhaust sensor from exhaust fumes would be useful, for example, in applications that require a clean air reference. What is needed in the art is a means for sealing the body of the exhaust sensor from exhaust fumes in a manner that conserves space while also assuring a durable seal.
The present invention is an exhaust sensor comprising: a shell; a talc assembly concentrically disposed within said shell, said talc assembly having a deformable talc disk; disposed between a first support disk and a second support disk, wherein at least one of said support disks is metal; and a sensor element disposed through said shell and said talc assembly; wherein said talc disk forms a seal against said shell and said sensor element.
The present invention also is a method for sealing an exhaust sensor comprising: positioning a talc assembly in a shell, said talc assembly comprising a talc disk disposed between support discs, wherein at least one of said support disks is metal; positioning an inner shield against the talc assembly; disposing a sensor element through said talc assembly; and applying force to the inner shield, thereby compressing the talc assembly and forming a seal between said talc assembly and said shell, and said talc assembly and said sensor element.